1. Field of the Invention
The present invention relates to screw anchors used to hold screws with respect to sheet metal parts, and more particularly to a screw anchor which is adjustable in the axial dimension so as to permit precise positional fitting between the sheet metal parts being fastened together by a screw threaded into the axially adjustable screw anchor.
2. Description of the Prior Art
The automotive industry has become increasingly quality conscious over the last several years. One of the most noticeable areas of increased quality has been the ever more precise fit between body parts, such as the fenders in relation to the hood and adjacent doors. In order to ensure a precision fit between the parts being fastened together in the face of unavoidable manufacturing variations, new fastener technologies have emerged which compensate for these manufacturing variations and thereby provide a precision fit of the parts being fastened together. Typically, final fit variations of no more than plus or minus 3.0 millimeters are considered acceptable.
Years ago, shims were used to adjust one part with respect to another. Shims require excessive labor involvement and are subject to imprecision.
More recently, a mill and drill pad fastener system has been developed, described in U.S. Pat. No. 4,438,971 to Zaydel et al, dated Mar. 27, 1984. In this system, pads are welded onto the frame, then the upper surface thereof is precision milled to provide an exact surface location for mounting thereupon the body part. Thereafter a drill hole is provided in the pad, and the body part then fastened to the pad using a common fastener. While this system provides excellent precision of fit with respect to body parts fastened to a frame, there is a considerable drawback in that the pads must be milled and a subsequent drill operation performed.
Still more recently, a form and pierce fastening system has been developed, described in U.S. Pat. No. 4,998,332 to Dacey, Jr., dated Mar. 12, 1991. In this system, a tool approaches the frame and forms a dimpled annular seat into the frame as well as pierces a hole at the base of the annular seat. The annular seat dimension varies from location to location, and is only as deep as is required to provide a predetermined seat for a spacing washer of a predetermined dimension, so that an end surface of the spacing washer provides a precise surface location for mounting thereupon the body part. A common fastener is then used to secure the body part to the hole in the frame at the spacing washer. This method provides the fit precision of the mill ad drill fastener system, yet is cheaper in operation. However, the form and pierce fastener system has drawbacks, too. One drawback is the considerable cost of the form and pierce tooling. Another drawback is that since the form and pierce tool must approach the frame, not all locations thereof are suitably located for access by the tooling. A variation on the form and pierce system is an emboss and pierce system. In this system a tool forms an embossment and hole on the frame, the embossment having a height which precisely provides a predetermined location for mounting the body part thereonto at the hole. This system has the same drawbacks as the form and pierce system, and has the additional problem of requiring tool access from behind the frame. In this regard, the frame upper rail onto which the left or right fender is attached must be provided with a large opening on the lower surface thereof so that access of the emboss and pierce tooling can be gained; this results in a weakened frame.
Accordingly, what remains needed in the art is a fastener system for providing precision mounting of body parts to a frame in spite of unavoidable manufacturing variations, wherein the fastener system is inexpensive, simple to use, universally applicable, and functionally reliable.